1. Field of the Invention
The present invention relates to a synchronization method between a reader and tag, in particular, to calculating a clock offset between the reader and tag and a message delay time, and thus synchronizing the reader with the tag or synchronizing the tag with the reader using the clock offset and the message delay time.
This work was supported by the IT R&D program of MIC/IITA. [2005-S-106-03, Development of Sensor Tag and Sensor Node Technologies for RFID/USN]
2. Description of Related Art
Recently, development of a semi-passive RFID tag containing a sensor which is available in a monitoring environment and its commercialization have been major issues. The semi-passive RFID tag containing the sensor enables a variety of application scenarios to be embodied. For example, the semi-passive RFID tag may be used for a sensor network.
The structural complexity of a sensor tag itself may be various depending on the accuracy of a clock of the sensor tag. That is, when the sensor tag simply monitors environment, such as a temperature, and is used for recording a deviation from a predetermined condition, the sensor tag may be designed to have a simple structure. That is, in this instance, accurate synchronization between the sensor tag and the reader is not required.
However, when the sensor tag stores an accurate time stamp together with environment information, synchronization of a local clock in the tag with a system clock of a reader is required. For example, when a product falls out of a production line, the local clock should be continuously synchronized with the system clock in order to accurately trace the time the product fell out of the production line.
Regularly synchronizing a clock is one of various synchronization methods between a tag and a reader in a conventional art. For example, every time sensing data is transmitted to a reader, synchronization between a tag and a reader may be operated. To achieve the above, a timer itself is treated as a register that is mapped to a specific region of a user memory so as to control a standard air interface through a command. Also, the timer is treated as a type of a special sensor so as to correct time information through an existing sensor access command.
However, the synchronization method between the tag and the reader according to the conventional art has a problem that it cannot guarantee accuracy. Accordingly, development of new technology which can accurately synchronize a reader and a tag is required.